Typically nuclear fission reactors for power generation are housed within a containment structure as a safety measure. Nuclear reactor containments are designed and employed to enclose the nuclear reactor pressure vessel containing the core of heat generating fissionable fuel and ancillary components of the system, such as portions of the coolant/heat transfer conduits or other means which constitute a source and/or means of conveyance of radiation and/or radioactive fission products. As such, the containment structure housing a nuclear reactor must effectively isolate the reactor system and components enclosed within its confines by sealing in all contents including any water, steam, gases or vapor and entrained fission products or other sources of radiation that may have escaped from the reactor pressure vessel and in particular its associated cooling system.
The provision of a construction fulfilling such requirements with an effective fluid impermeable confinement structure securely isolating its enclosed contents from the external atmosphere does not generally present either a significant engineering or construction obstacle or achievement.
However, in the event of certain malfunctions in a nuclear reactor system, such as a loss of coolant, large volumes of very hot pressurized water may be released from the system into the interior of the containment structure. This very hot pressurized water flashes into steam which may carry along radioactive fission products, and substantially increase the pressure and temperature within the containment structure. Such accidents can produce very high pressures and temperatures within the confines of the "leak proof" containment structure thereby imposing heavy demands upon its integrity and ability to perform its designed role of retaining all potentially hazardous matter derived from the nuclear reactor system.
Potentially deleterious high pressure due to the inherent high thermal energy and flashing steam cannot simply be released by venting from the containment or otherwise permitted to escape to the outside atmosphere since the steam vapor may entrain and carry radioactive fission products which would also be released into the environment.
A variety of suppression schemes have been proposed and devised to cope with the problem of excessive pressure. They include a variety of measures or arrangements for condensing evolving or flashing steam and reducing the resultant over-pressure caused by accidents, for example, the designs disclosed in U.S. Pat. Nos. 3,713,968; 4,362,693; 4,473,528; and 4,526,743.
In the absence of an effective suppression means to mitigate steam generated high pressures, the enveloping containment structure must be designed and constructed at excessively high costs and maintained to resist and retain inordinately high internal fluid pressures. Nevertheless, even a significantly reinforced containment structure cannot be assured to be resistant to breaching considering the temperature/pressure potential of a typical power generating nuclear reactor plant.
Under malfunctioning conditions, the decay heat produced by the core of fissionable fuel within the pressure vessel is released into the containment via either a pressure suppression vent system or the pressure vessel safety and depressurization valves. In conventional reactor assemblies this excess thermal energy is commonly removed by active cooling systems comprising motors, pumps, valves and heat exchangers. Functioning of these acting components depends upon external power and/or proper operator personnel actions.